The invention relates to a switched-mode power supply comprising a transformer having a primary winding connected to a first pair of direct voltage terminals in a primary circuit via a first controllable switching element and having a secondary winding included in a secondary circuit via a unidirectional element and a load to be powered.
A switched-mode power supply of the type defined in the opening paragraph is known from U.S. Pat. No. 4,652,984 and can be used for charging batteries and for powering electrical equipment. Such a power-supply circuit is particularly suitable for an electric shaver having rechargeable batteries, the power-supply circuit supplying the charging current for the batteries and the current for powering the motor of the shaver. Said Patent discloses a flyback-type self-oscillating switched-mode power supply having a transformer. The transformer permits an adaptation between the comparatively low voltage of the rechargeable battery and/or of the shaver motor and the comparatively high rectified mains voltage. The duty cycle of the first controllable switching element, generally a switching transistor, can thus remain within practical limits. It is possible to operate this known switched-mode power supply with a lower input voltage, for example 12 V d.c. or 24 V d.c., instead of the normal mains voltage though, on account of the transformation ratio, the maximum possible charging current for the rechargeable battery is smaller than attainable in the case of a transformation ratio adapted to this end.
In the International Application No. WO 95/01000 this problem is remedied by means of a switched-mode power supply which uses a transformer having two primary windings, each having a separate switching element. The associated primary winding and switching element are put into operation depending on the applied input voltage, i.e. the rectified mains voltage or 12 V direct voltage. This results in an optimum adaptation for the different input voltages. However, a drawback of this solution is that it requires a transformer having two separate primary windings, which complicates the design of the transformer.